Ask HN: How do you track and generate line items when invoicing?

This is probably a question best posed to freelancers; when you are working for clients, how do you track what will eventually become the line items in your invoice? Or maybe you don't include a detailed breakdown of tasks completed in invoice? If you don't, why not?

For example, as a web developer I was finding that when it came to invoicing a client (every 2/4 weeks) I would have a big empty expensive invoice with no indication of what was done. I first tried to simply log manually the tasks that I finished and on what date but I invariably forgot to add them sometimes. I then started using my git commit log to get an overview of what I did in a days work for a client. I use these commit messages to create an invoice with each line item containing the relevant commit messages for each day.

Is this an issue for anyone else? If not, why not? If so, how do you deal with it?